The present invention relates to electrolytic capacitors containing a device for sorbing the harmful substances created by such capacitors during their use. The present invention also relates to a method for sorption of the substances.
Known electrolytic capacitors, e.g. EDLC supercapacitors (Electrochemical Double Layer Capacitor), are essentially comprised of an airtight housing, wherein electrodes typically formed of metal sheets are arranged, the electrodes being immersed in particular electrolytic solutions. The housing also contains gettering elements for sorption of harmful substances, and electrical contacts communicating the electrodes with the outside of the capacitor.
The electrolytic solutions are typically formed of a solvent and an ionic salt. In the EDLC case, for example, acetonitrile and propylene carbonate are frequently employed as solvents, while tetraethylammonium tetrafluoroborate is often used as a salt.
During use, these solutions can create harmful substances, often in gaseous form, which can damage the capacitors, possibly in an unrepairable manner. Another possible source of harmful gases can be due to the desorption of some materials used inside the capacitor.
Carbon dioxide, carbon monoxide, and hydrogen are among the most harmful gaseous species; while water, which is another particularly harmful species, is typically present in liquid form inside the electrolytic solution.
The problem of the sorption of harmful species inside the capacitors can be tackled by adding one or more sorbing elements mixed in the electrolytic solution, or by non-mixed sorbing systems. The use of materials with a gettering action mixed in the electrolyte can be accomplished by liquid sorbers. This solution is disclosed, for example, in International patent application PCT/IT2006/000349 of SAES Getters S.p.A. A second embodiment prescribes the use of solid sorbers added to the electrolyte, as described in Japanese patent application publication JP 03-292712, wherein an additive including a particulate of platinum, palladium or alloys thereof is applied onto the sheets after these have been impregnated with the electrolytic solution. However, these sheets may have a very small thickness, in particular less than 10 micrometers (μm), whereby the sheets may be damaged by the particles contained in such a particulate, due to their relatively large diameter, resulting in the risk of accidental short-circuits within the capacitor.
A system employing gas sorbers placed in delimited regions of the capacitor is described in Japanese patent application publication JP 2003-197487. In this case, the sorbing material is used in the form of sheets of a polymeric material, such as polypropylene, as a support of the sorbing material. These sheets are directly contacted by the electrolytic solution.
These types of solution are limited by the fact that the sorbing material, in addition to having the function of sorbing the harmful substances produced within the capacitor, must be compatible with the electrolyte, i.e., it must be completely inert with respect thereto, in order to prevent its sorbing properties from being jeopardized, or even worse, in order to prevent chemical species harmful to the correct operation of the capacitor being released as an effect of the reaction with the electrolyte. For example, the possible decomposition of the gas sorber could vary the electric conductivity of the electrolyte. Such a compatibility must be guaranteed by the sorber, even after the sorber has carried out its function by binding with the harmful species.